8A.4 OBSERVATIONS OF GULF OF TEHUANTEPEC GAP WIND EVENTS FROM QUIKSCAT: AN UPDATED EVENT CLIMATOLOGY AND OPERATIONAL MODEL EVALUATION Michael J. Brennan*, Hugh D. Cobb, III, and Richard D. Knabb NOAA/NWS/NCEP/National Hurricane Center 1. INTRODUCTION QuikSCAT satellite, launched in 1999, has allowed As part of the National Hurricane Center (NHC), the consistent and unprecedented documentation of Tropical Analysis and Forecast Branch (TAFB) has these events, including their frequency, duration and marine analysis and forecast responsibility for intensity (primarily gale vs. storm force). portions of the eastern tropical Pacific Ocean (Fig. 1). Gap wind events in the Gulf of Tehuantepec are the NRT QuikSCAT retrievals have been available to most frequently observed storm-force (48 kt or forecasters at NHC since the fall of 1999, allowing for greater) wind events that occur outside of tropical routine observation of these events and their intensity cyclones in this region. for the first time, the evaluation of operational model forecast performance in these events, and the The Gulf of Tehuantepec is situated south of the development of a multi-year event climatology. Isthmus of Tehuantepec, the narrowest area of land separating the Gulf of Mexico from the Pacific Ocean An initial climatology of Tehuantepec events was (Fig. 2). When cold-air outbreaks occur over the Gulf constructed by Cobb et al. (2002) based on the first of Mexico, the Sierra Madre Mountains block the three cold seasons of data (1999–2000 through large-scale flow of cold air southward, resulting in the 2001–2002) from the QuikSCAT mission. That study development of a strong north-south pressure found an average of 15 gale-force and two storm- gradient across the isthmus. Subsequently, northerly force events occurred each cold season. The first flow is funneled through the 40-km-wide Chivela Pass, gale-force event of a season typically occurred in mid- a narrow break in the Sierra Madres. This gap flow October, with the final gale-force event occurring in often results in a narrow jet of winds that can reach late March or early April. gale (≥ 34 kt), storm (≥ 48 kt), and occasionally hurricane force (≥ 64 kt) in the Gulf of Tehuantepec The goals of this study are (i) to utilize the long period (Fig. 3). of data from QuikSCAT to update the initial climatology of Cobb et al. (2002), and (ii) to examine From previous case studies (e.g., Schultz et al. 1997; the performance of real-time NWP model guidance Steenburgh et al. 1998), and climatological studies available to TAFB forecasters for predicting these (e.g., Schultz et al. 1998), the synoptic-scale features events during the 2006–2007 cold season. driving cold-air surges that result in gap wind events in the Gulf of Tehuantepec are relatively well- A brief description of QuikSCAT will be provided in understood. The strongest cold surges into the region section 2, with the updated climatology presented in are associated with a confluent upper-level jet section 3. Section 4 contains an evaluation of entrance region situated over the Gulf of Mexico and operational NWP model guidance for the 2006–2007 an upper-level ridge over the western United States, cold season. Section 5 will discuss other potential resulting in the equatorward movement of a surface sources of OSVW data for the detection of these anticyclone of arctic origin into the Gulf of Mexico events in the post-QuikSCAT era and section 6 will (Schultz et al. 1998, their Fig. 14a). present a summary and conclusions. Prior to the advent of satellite-based wind 2. QUIKSCAT DESCRIPTION observations, sporadic ship observations provided the only information on the intensity of these gap wind The SeaWinds scatterometer onboard QuikSCAT is a events over the open waters of the Gulf of Ku-band scatterometer that estimates OSVW by Tehuantepec, and little was known about the spatial measuring the backscatter due to centimeter-scale distribution of the winds in these events. More capillary waves on the ocean surface. QuikSCAT recently, the availability of near-real-time (NRT) ocean nominally provides wind retrievals with a horizontal surface vector wind (OSVW) retrievals from the resolution of 25-km, and since 2003 NRT 12.5-km SeaWinds scatterometer onboard the NASA retrievals have also been available. The NRT QuikSCAT retrievals available at NHC are processed *Current affiliation: NOAA/NWS/NCEP at the National Oceanic and Atmospheric Hydrometeorological Prediction Center Administration/National Environmental Satellite, Data, Corresponding author address: and Information Service (NOAA/NESDIS) using the Dr. Michael J. Brennan NRT retrieval process described by Hoffman and 5200 Auth Rd. Leidner (2005). These data are displayed on the Camp Springs, MD 20746 NOAA/National Centers for Environmental Prediction Email: [email protected] (NCEP) Advanced Weather Interactive Processing System (N-AWIPS) workstations used by NHC events. The increased number of storm events during forecasters. the 2006–2007 cold season occurred after the positive ENSO event in 2006. This is consistent with QuikSCAT provides wind retrievals in an 1800-km- the findings of Romero-Centeno et al. (2003), who wide swath, often twice per day near the Gulf of showed an increase in the frequency and strength of Tehuantepec. At 16°N (the approximate latitude of northerly winds in the southern Isthmus of the Gulf of Tehuantepec), the gap between adjacent Tehuantepec during ENSO years, and Schultz et al. QuikSCAT swaths is approximately 930 km wide, (1998), who showed that Central American cold resulting in occasional “misses” when the Gulf of surges were more numerous after an El Nino year, Tehuantepec area falls in these gaps. Conveniently due to a more prominent jet entrance region over the for synoptic analysis and comparison to NWP model Gulf of Mexico. output, QuikSCAT passes in the Gulf of Tehuantepec region generally occur within an hour of 00 UTC and The monthly distribution of Tehuantepec events 12 UTC. shows the largest number of gale-force events occurring in December (Fig. 5). Storm-force events Confidence in the accuracy of QuikSCAT OSVW occur most often in January; with a secondary peak in retrievals is high for Tehuantepec events, especially November. Gale-force events have occurred as early since these wind events are rain-free, thus eliminating as September and as late as May, however about rain contamination as a source of error in the 88% of the events occur between November and QuikSCAT retrievals. Recent measurements of March. hurricane-force winds in extratropical cyclones in non- raining conditions using the NOAA Stepped- 4. OPERATIONAL MODEL EVALUATION Frequency Microwave Radiometer (SFMR) and GPS dropsondes were in close agreement with co-located TAFB forecasters include forecasts and warnings for QuikSCAT measurements (P. Chang, personal Tehuantepec events in a high seas forecast issued communication); confirming that QuikSCAT OSVW four times daily. NWP model guidance available in retrievals are accurate in the low end of the hurricane- the region includes various global models, the NCEP force range in non-raining conditions. Global Forecast System (GFS) being the most heavily used. For the first time in 2006–2007, the NCEP 3. TEHUANTEPEC EVENT CLIMATOLOGY North American Mesoscale (NAM) model extended far enough south to include the Gulf of Tehuantepec For each cold season from 1999–2000 through 2006– region. Here operational model guidance for 2007, all QuikSCAT passes showing winds of gale Tehuantepec events during the 2006–2007 cold force or greater in the Gulf of Tehuantepec were season is evaluated. cataloged. The classification of an event as gale- or storm-force is based on the peak QuikSCAT wind At the time of this study, the GFS model was run with observed during the event from either 25-km or 12.5- horizontal spectral truncation at wave number 382 (an km retrievals. Since 12.5-km QuikSCAT retrievals effective horizontal grid spacing of approximately 40 only became available in 2003, the number of storm km) and 64 vertical levels (T382/64) through the first events detected has increased substantially. Since 180 hours of the forecast, while the NAM model was the mean duration of a Tehuantepec event is 48 run with 12-km horizontal grid spacing and 60 vertical hours (Cobb et al. 2002), one or two QuikSCAT levels. GFS output is delivered to TAFB forecasters passes that “miss” during an event would not likely on global 1° grids, and output from the NAM is preclude detection of a gale-force event, but could provided on grids with approximate 90-km horizontal result in missing a storm-force event, since those grid spacing. peak winds only occur during a short portion of the overall event. For all times when QuikSCAT indicated at least gale- force winds in the Gulf of Tehuantepec, the maximum During the 1999–2000 through 2006–2007 period, 99 forecast wind speed from the GFS and NAM (to the gale- and 44 storm-force events have been observed; closest 5 kt) was recorded at 12, 24, 36 and 48 h lead an average of 12.4 gale and 5.5 storm events per cold times. Table 1 summarizes the number of gale- and season. Since the advent of 12.5-km retrievals in storm-force events, as observed by QuikSCAT, for 2003, the number of storm-force events detected has which forecasts from each model were evaluated. increased to an average of 7.4 per season from The maximum QuikSCAT wind speed was obtained 2002–2003 through 2006–2007. The yearly count of from gridded analyses with 1/4° (1/8°) grid spacing for gale- and storm-force events is shown in Figure 4. the 25-km (12.5-km) retrievals 1 .The probability of The 2003–2004 cold season had the largest number detection (POD) of a gale-force or a storm-force event of events reach at least gale force (24), while the (as determined by QuikSCAT) from both the GFS and 2006–2007 cold season had the most storm-force NAM was computed at 12-, 24-, 36- and 48-h lead events (12).